ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Swapping Preamp Tubes

4/20/2000 2:18 PM
Ray Ivers	Swapping Preamp Tubes Hello to All, First off, my name is Ray Ivers, and I've got a small company here in Connecticut (R.A.G.E. Electronics), modding and repairing guitar amps since '84. I'm new to Ampage, and so far I've found it to be an incredible forum for guitar amp knowledge. I'll try not to screw it up. I've been seeing references to swapping preamp tubes in amps for many years, and until recently I thought it was a cool thing to do. I no longer think that. I realize that 'if it sounds good, it is good', but it's possible that the person who plugs a 12AU7 into a 12AX7 circuit thinks that he's hearing the sound of a properly-operating 12AU7. He (or she) is not! I've read that 'preamp tubes do not require biasing', but in my opinion this should read 'preamp tubes do not require biasing when replaced with the original tube type and/or if you agree with the original biasing of the tube'. How many of you agree with the original biasing of your power tubes, from the factory? Granted, this is a design issue and not a tweak, but lots of amps are designed with non-adjustable fixed-bias output stages, something I've never considered a good idea. Pluggin a 12AU7 into a Marshall's first gain stage (100K plate resistor, 2.7K cathode resistor, approximately 135 plate volts) results in a current flow of less than 2 mils thru the 12AU7. This means that the 12AU7 is operating in a very non-linear part of its characteristic curves. There are circuits that use devices operating in this region to convert triangle waves to sine waves! This may be a cool effect, but clean it ain't. To properly use a 12AU7 (or 12AT7, or most other 12A_7 tubes) in a circuit designed for another tube will require at the very least a change of the cathode resistor value (along with bypass cap, if any, to maintain the same frequency response), and in the cases of a radical type-change like the 12AU7-for-12AX7 swap mentioned above, a change of plate resistor as well. This is necessary to maintain symmetrical signal-swing and maximum output voltage as well as low distortion, assuming that's what you want. Most guitar amps seem to run the preamp tubes at low currents, whereas hi-fi amps usually run tube currents higher (the 12AX7 runs on super-low current, and is kind of a different animal from the others, although current flow through a 12AX7 will still affect tone). My guess is that users of amps with 12AT7 driver tubes would like the sound of their amp better with higher-current operation of that driver tube - the gain increases slightly but more importantly, the output impedance of the driver stage drops so you can hit the output tubes a little harder. Most people don't realize that most of what is called 'output tube distortion' is not that at all - it's your driver tube clipping when it finds it's just not up to the task of operating the output stage in Class A2 or AB2, where the grids are driven positive with respect to ground (i.e. a 46 volt peak signal applied to a grid with -45 volts bias). At any rate, probably no harm will be done to the amp's circuitry by swapping different tube types, but tube life can be greatly affected by it, especially in the Fender reverb drive stage. There's a lot of potential for getting different sounds out of your amp by swapping preamp tubes, but it's not quite as cut-and-dried as plug'nplay. C'mon - admit it - didn't it always seem just a little too good to be true? And power tubes - that's a whole 'nother story. I would welcome any comments on this subject, and also I should add that there are plenty of tubes that CAN be substituted freely (7025/ECC83/5751/12AX7, etc.). Ray Ivers

4/20/2000 2:18 PM

Ray Ivers

Swapping Preamp Tubes

Hello to All,

First off, my name is Ray Ivers, and I've got a small company here in Connecticut (R.A.G.E. Electronics), modding and repairing guitar amps since '84. I'm new to Ampage, and so far I've found it to be an incredible forum for guitar amp knowledge. I'll try not to screw it up.

I've been seeing references to swapping preamp tubes in amps for many years, and until recently I thought it was a cool thing to do. I no longer think that. I realize that 'if it sounds good, it is good', but it's possible that the person who plugs a 12AU7 into a 12AX7 circuit thinks that he's hearing the sound of a properly-operating 12AU7. He (or she) is not! I've read that 'preamp tubes do not require biasing', but in my opinion this should read 'preamp tubes do not require biasing when replaced with the original tube type and/or if you agree with the original biasing of the tube'. How many of you agree with the original biasing of your power tubes, from the factory? Granted, this is a design issue and not a tweak, but lots of amps are designed with non-adjustable fixed-bias output stages, something I've never considered a good idea.

Pluggin a 12AU7 into a Marshall's first gain stage (100K plate resistor, 2.7K cathode resistor, approximately 135 plate volts) results in a current flow of less than 2 mils thru the 12AU7. This means that the 12AU7 is operating in a very non-linear part of its characteristic curves. There are circuits that use devices operating in this region to convert triangle waves to sine waves! This may be a cool effect, but clean it ain't. To properly use a 12AU7 (or 12AT7, or most other 12A_7 tubes) in a circuit designed for another tube will require at the very least a change of the cathode resistor value (along with bypass cap, if any, to maintain the same frequency response), and in the cases of a radical type-change like the 12AU7-for-12AX7 swap mentioned above, a change of plate resistor as well. This is necessary to maintain symmetrical signal-swing and maximum output voltage as well as low distortion, assuming that's what you want. Most guitar amps seem to run the preamp tubes at low currents, whereas hi-fi amps usually run tube currents higher (the 12AX7 runs on super-low current, and is kind of a different animal from the others, although current flow through a 12AX7 will still affect tone). My guess is that users of amps with 12AT7 driver tubes would like the sound of their amp better with higher-current operation of that driver tube - the gain increases slightly but more importantly, the output impedance of the driver stage drops so you can hit the output tubes a little harder. Most people don't realize that most of what is called 'output tube distortion' is not that at all - it's your driver tube clipping when it finds it's just not up to the task of operating the output stage in Class A2 or AB2, where the grids are driven positive with respect to ground (i.e. a 46 volt peak signal applied to a grid with -45 volts bias).

At any rate, probably no harm will be done to the amp's circuitry by swapping different tube types, but tube life can be greatly affected by it, especially in the Fender reverb drive stage.

There's a lot of potential for getting different sounds out of your amp by swapping preamp tubes, but it's not quite as cut-and-dried as plug'nplay. C'mon - admit it - didn't it always seem just a little too good to be true? And power tubes - that's a whole 'nother story.

I would welcome any comments on this subject, and also I should add that there are plenty of tubes that CAN be substituted freely (7025/ECC83/5751/12AX7, etc.).

Ray Ivers

4/20/2000 2:50 PM
Rebel420	Ray, first off, welcome aboard. I've been following some of your posts and suspected you were someone who did this as more than just a hobby Your opinions on swapping preamp tubes is definiely one that is making me think... and itching to do some more playing tonite when i get home ....

4/20/2000 4:06 PM
Ray Ivers	Thanks - and thanks also for the 'brown sound' info. So many times I'm asked to get a certain sound from an album, and don't know what to say: "I can't really do that, and here's why" gets old real quick. Sounds to me from your postings that you're no stranger to design-level stuff, either. Yeah, after I made that last posting I kind of wondered if I should have; but I'd be glad to figure out what cathode/plate resistor values I feel would be good for anyone who wants to ask me. I just got thru with my own Ampeg V-9 (SVT for guitar), redesigning the negative-headroom driver stage to obtain full output while maintaining as many original parts as possible (ended up moving only one resistor!). I really enjoy doing this, maybe more than I should. So much focus is put on preamp tubes' gain factor, when there are so many other parameters that affect tone and power output - that's why there are so many tube types, and a lot of times a 12AX7 is NOT the best choice for a given circuit, as you well know. A cool multi-purpose 12AX7 circuit, though, is the Marshall/tweed Bassman tone stack driver circuit: a 12AX7 gain stage direct-coupled to a cathode follower (the 30th Anniversary has three of these 'stagelets' in its 4-stage overdrive channel; it's why I bought the amp, and I think it sounds great) - I've got a paper by T.E. Rutt for the AES convention about this circuit... See what I mean? I have to get a life! Just kidding - I eat, sleep, and breathe this stuff, and wouldn't have it any other way. Ray

4/20/2000 4:06 PM

Ray Ivers

Thanks - and thanks also for the 'brown sound' info. So many times I'm asked to get a certain sound from an album, and don't know what to say: "I can't really do that, and here's why" gets old real quick. Sounds to me from your postings that you're no stranger to design-level stuff, either.

Yeah, after I made that last posting I kind of wondered if I should have; but I'd be glad to figure out what cathode/plate resistor values I feel would be good for anyone who wants to ask me. I just got thru with my own Ampeg V-9 (SVT for guitar), redesigning the negative-headroom driver stage to obtain full output while maintaining as many original parts as possible (ended up moving only one resistor!). I really enjoy doing this, maybe more than I should.

So much focus is put on preamp tubes' gain factor, when there are so many other parameters that affect tone and power output - that's why there are so many tube types, and a lot of times a 12AX7 is NOT the best choice for a given circuit, as you well know. A cool multi-purpose 12AX7 circuit, though, is the Marshall/tweed Bassman tone stack driver circuit: a 12AX7 gain stage direct-coupled to a cathode follower (the 30th Anniversary has three of these 'stagelets' in its 4-stage overdrive channel; it's why I bought the amp, and I think it sounds great) - I've got a paper by T.E. Rutt for the AES convention about this circuit...

See what I mean? I have to get a life! Just kidding - I eat, sleep, and breathe this stuff, and wouldn't have it any other way.

Ray

4/20/2000 4:56 PM

Rebel420

Wellth brown sound info was truely my opinion and speculation... I can agree with you, tho, you are probably tired of every 16 yr old kid coming in with a JCM900 or Peavy 5150 wanting to get the old VH sound, and then thinking you are an ass because their amp alone will not deliver it.
On a design level, well I'm fairly new at that, I've done a number of restorations over the past 5 or so years, did a little of this and that between hs and that point, and when in JR. high and HS, i built a lot of effects. The design is something that i started getting into over the past year or so, so like i said, i'm still green there... A pointer I can give to anyone out there, is build a champ clone. a very simple amp, tha tcan be a great testbed for doing comparisons. I decided to do that after seeing too many debates about carbon vs metal resistors etc. I dont do a LOT of the math, a lot of what I do is gut feeling and intuition, and a bit of random luck involved *laughs*
I agree that an 'ax7 may not always be the best choice.. but a lot of times they are used for sheer marketing purposes, buy in bulk, and hey, a 1k resistor costs the same as a 5.6k, so hey we can alwasy tweak the gain down...

quote:
"See what I mean? I have to get a life! Just kidding - I eat, sleep, and breathe this stuff, and wouldn't have it any other way."

Heh, ido that in the midst of working a full time job in the datacom field... *laughs* but I understand that completely...

4/20/2000 5:11 PM
dale	ray, your comments make sense to me from what i have experienced. i always felt that when swapping a u7 or t7 for an x7 in my marshalls, even if they did change the tone in the way i wanted, it just never seemed quite right. like something wasn't letting the amp deliver the quality of tone i'm used to in the amp. the biasing of the pre tube would explain this. so i would venture to guess you're correct about this. dale

4/20/2000 5:11 PM

dale

ray,

your comments make sense to me from what i have experienced. i always felt that when swapping a u7 or t7 for an x7 in my marshalls, even if they did change the tone in the way i wanted, it just never seemed quite right. like something wasn't letting the amp deliver the quality of tone i'm used to in the amp. the biasing of the pre tube would explain this. so i would venture to guess you're correct about this.

dale

4/20/2000 6:47 PM
Ray Ivers	Dale, The good news is that it's usually pretty easy to optimize for the new tube type, especially if the correct value (perhaps I should say 'the range of correct new values') of cathode resistor required is less than the existing one, which would generally be the case if you were going from a 12AX7 to a 12AT7/12AU7 (more current required, less bias voltage needed, smaller resistor value). This way, you could easily just plug in the new tube, clip-lead the new resistor that in parallel with the existing resistor would result in the desired total cathode resistance and bias voltage, and try out the sound; if you didn't like it, no harm no foul - just unclip and you're back again. I use the TubeCAD program to find the values and model the circuit, and it's never let me down yet. If I can ever help you out to find a new resistor value, let me know. Ray

4/20/2000 6:47 PM

Ray Ivers

Dale,

The good news is that it's usually pretty easy to optimize for the new tube type, especially if the correct value (perhaps I should say 'the range of correct new values') of cathode resistor required is less than the existing one, which would generally be the case if you were going from a 12AX7 to a 12AT7/12AU7 (more current required, less bias voltage needed, smaller resistor value). This way, you could easily just plug in the new tube, clip-lead the new resistor that in parallel with the existing resistor would result in the desired total cathode resistance and bias voltage, and try out the sound; if you didn't like it, no harm no foul - just unclip and you're back again.

I use the TubeCAD program to find the values and model the circuit, and it's never let me down yet. If I can ever help you out to find a new resistor value, let me know.

Ray

4/20/2000 9:28 PM
Dave James	Again, welcome. I too have followed your posts with interest. And don't worry about relating seemingly "touchy" topics. I especially liked the term "anti-hum" in the "Hum...Great Tone" posts. I think most of us would agree that the "plug-and-play", swap of 12A_7 tubes is not completely valid in circuit terms. Looking at the tube charts surely would show this. But this is where we get into the "sounds good, must be good" mode. And THAT is how fables are made (grins). Later, DJ

4/20/2000 9:28 PM

Dave James

Again, welcome. I too have followed your posts with interest. And don't worry about relating seemingly "touchy" topics. I especially liked the term "anti-hum" in the "Hum...Great Tone" posts.

I think most of us would agree that the "plug-and-play", swap of 12A_7 tubes is not completely valid in circuit terms. Looking at the tube charts surely would show this. But this is where we get into the "sounds good, must be good" mode.

And THAT is how fables are made (grins).

Later,

DJ

Page 1 of 2

Next>

Last Page>>